1. Field of the Invention
This invention relates to digital voice communication and in particular to packet switched digital voice and data communication. More particularly, the invention relates to packet switched digital voice and data communication over a network capable of handling high data rates, including media such as digital trunks used in the telephone network or coaxial cable as used in cable television networks.
2. Description of the Prior Art
Packet communication is known in general. However, known packet switching techniques typically involve variable and arbitrary delay in a store and forward environment. These delays are unacceptable for two-way communication involving real-time voice signals. The human ear is critically sensitive to absolute delay greater than about 200 ms. The ear is also critically sensitive to random delays or gap modulation.
Bell-Northern Research has developed a digital concentrator system under the designation PLC-1 to connect two distant points of a private network over a telephone trunk. An article by Black et al., "PLC-1: Digital Technology Cuts Cost of Analog Trunking," Telesis 1982 Three pages 2-8 describes digital speech interpolation in which systems allocate signal transmission according to loading rules. Digital speech interpolation (DSI) techniques employ digital signal processing techniques and the statistics of speech for detecting signal load and for adjusting to channel capacity. In the PLC-1 system, one load is handled by temporarily storing speech bursts until the momentary overload disappears. Such a technique may result in delays of more than 300 ms (1/3 sec) in about 10% of the occurrences of overload. Up to twice normal channel capacity may be achieved.
Aydin Monitor Systems of Fort Washington, Penna. offers a T-1 channel voice data multiplexer capable of multiplexing 48 voice channels into a single T-1 channel normally designed to handle 24 voice channels. The Aydin system employs a variable quantizing level (VQL) technique which allows two to one voice compression to increase the effective channel capacity of the channel with subjectively minimal degradation of voice quality. A data sheet dated July 1982 describes such a channel bank facility.
Aydin Monitor Systems has recently announced a T-1 channel voice/data multiplexer for which it is claimed 72 voice or data channels can be handled over a standard T-1 trunk. The new system is understood to use VQL encoding and digital speech interpolation (DSI) techniques.
The literature of packet voice transmission is extensive and suggests that packet voice transmission is not very cost effective or of high quality. A representative tutorial article on packet voice is "Packet Voice: When It Makes Sense" by Randy Cole in the September/October, 1982 issue of Speech Technology. The present invention seeks to overcome the limitations noted in the literature.
Time division multiplexing (TDM) systems are known including the time division multiplexed pulse code modulation systems associated with the T-1 carrier system used in the U.S. PCM communication is based on circuit switching in which analog voice signals are converted to digital signals and then interlaced in the time slots with other similar signals. In TDM, 24, 30 or even more device signals are interlaced in time and passed through a telephone transmission system at a bit rate of generally 1.544 or 2.048 Mbps. The bit stream occurs as a result of sequentially scanning samples of each of 24 analog to digital converters per each timing frame. Each of the 24 time slots in the frame transmits a separate circuit. In a T1 PCM scheme, it is possible to perform some limited circuit switching by slightly delaying the incoming bit stream to place a frame in a selected time slot. Command signaling which controls the delay may be transmitted in connection with the information bit stream although out of band. With conventional circuit switching techniques, the ratio between the transmission time and the switching period is very long. PCM lends itself both to time division switching and to conventional matrix space division switching. (Time division switching is switching wherein each time slot represents a different path. Space division switching is switching wherein each input path is switched to a separate outgoing path.)
All of the systems hereinabove described relating to conventional telephone systems are channel bank architectures which are operative to convert individual analog telephone voice channels to digital pulse code modulation (PCM) signals on a channel by channel basis. The systems are thus limited to analog/digital conversion with point to point signal direction.